MCAT Biology : Other Cell Structures

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #1 : Other Cell Structures

In which of the following pH environments would an enzyme from the small intestine most likely be active?

Possible Answers:

pH = 5

pH = 7

pH = 9

pH = 3

pH = 12

Correct answer:

pH = 7

Explanation:

Chyme from the stomach is transferred to the small intestine through the pyloric sphincter. The highly-acidic chyme is then neutralized in the duodenum of the small intestine. The majority of the small intestine has a pH between 6 and 7.

Example Question #1 : Other Cell Structures

Which of the following is NOT a function of peroxisomes?

Possible Answers:

Metabolism of hydrogen peroxide

Breakdown of very long chain fatty acids

Production of plasmalogen

Storage of lipase

Correct answer:

Storage of lipase

Explanation:

Peroxisomes are involved in the metabolism of hydrogen peroxide and very long chain fatty acids. They are also known to produce plasmalogen, an important phospholipid found in myelin, without which disorders of the nervous system can arise. Lysosomes, not peroxisomes, store acid hydrolases such as lipase.

Example Question #3 : Other Cell Structures

Most scientists subscribe to the theory of endosymbiosis to explain the presence of mitochondria in eukaryotic cells. According to the theory of endosymbiosis, early pre-eukaryotic cells phagocytosed free living prokaryotes, but failed to digest them. As a result, these prokaryotes remained in residence in the pre-eukaryotes, and continued to generate energy. The host cells were able to use this energy to gain a selective advantage over their competitors, and eventually the energy-producing prokaryotes became mitochondria.

In many ways, mitochondria are different from other cellular organelles, and these differences puzzled scientists for many years. The theory of endosymbiosis concisely explains a number of these observations about mitochondria. Perhaps most of all, the theory explains why aerobic metabolism is entirely limited to this one organelle, while other kinds of metabolism are more distributed in the cellular cytosol.

The failure of phagocytosis in the initial stages of endosymbiosis could have been due to the failure of host cells to produce digestive enzymes. Most cells generate digestive enzymes into the vesicle that houses newly-phagocytosed material, or phagosome. Failure to form which of the following would be expected in a cell that was unable to digest compounds in its phagosome?

Possible Answers:

Ankyrin

Cell membrane receptors

Cell membrane transporters

Lysosome

Spectrin

Correct answer:

Lysosome

Explanation:

Lysosomes are the cell's repository of digestive enzymes. When these merge with phagosomes, the result is a vesicle that is able to digest phagocytosed material.

Spectrin and ankyrin are structural proteins associated with the cytoskeleton, especially in muscle cells.

Example Question #2 : Other Cell Structures

One component of the immune system is the neutrophil, a professional phagocyte that consumes invading cells. The neutrophil is ferried to the site of infection via the blood as pre-neutrophils, or monocytes, ready to differentiate as needed to defend their host.


In order to leave the blood and migrate to the tissues, where infection is active, the monocyte undergoes a process called diapedesis. Diapedesis is a process of extravasation, where the monocyte leaves the circulation by moving in between endothelial cells, enters the tissue, and matures into a neutrophil.

Diapedesis is mediated by a class of proteins called selectins, present on the monocyte membrane and the endothelium. These selectins interact, attract the monocyte to the endothelium, and allow the monocytes to roll along the endothelium until they are able to complete diapedesis by leaving the vasculature and entering the tissues.

The image below shows monocytes moving in the blood vessel, "rolling" along the vessel wall, and eventually leaving the vessel to migrate to the site of infection.

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A scientist is investigating the digestive enzymes present in a neutrophil as it digests foreign material. Where are these enzymes most likely to be stored immediately before they are used to break down phagocytosed material?

Possible Answers:

Lysosome

Mitochondria

Nucleus

Endoplasmic reticulum

Golgi body

Correct answer:

Lysosome

Explanation:

The lysosome is the main storage site for digestive proteins. These digestive proteins are then released into the phagosome to form the phagolysosome. This phagolysosome is then able to digest phagocytosed material and render it harmless to the host.

Example Question #3 : Other Cell Structures

Which of the following scenarios could directly cause cell death?

Possible Answers:

Presence of hydrogen peroxide in the peroxisome

Disruption of the lysosome and release of its contents

A silent mutation in a DNA nucleotide base

Overstimulation of the cell cycle

Inactivation of a transport protein in the membrane

Correct answer:

Disruption of the lysosome and release of its contents

Explanation:

The lysosome contains hydrolytic enzymes that break down biological materials. If these enzymes were released, they could cause cell death by autolysis.

The inactivation of a transport protein would not be too catastrophic, and would not lead to cell death. Overstimulation of the cell cycle would lead to increased mitosis and cell proliferation, the opposite of cell death. Eventually, this overstimulation may be detected and cause apoptosis, but it will not directly cause the cell death. A silent mutation would not alter the protein created from the gene, and thus would have no effect on the cell.

Example Question #6 : Other Cell Structures

Which of the following may be the pH inside a lysosome?

Possible Answers:

Correct answer:

Explanation:

Generally, a lower pH is best for lysosomal degradation. Lysosomes contain hydrolytic enzymes, which will only function in the presence of an acid catalyst. The best answer choice is a pH of 4, which is slightly acidic but not as acidic at a pH of 1. Even the stomach, which is considered extremely acidic, only has a pH of about 2.5. Lysosomal degradation at a pH of 1 would cause even the membrane integrity to be questionable.

Example Question #7 : Other Cell Structures

There are two models for the operation of the Golgi apparatus in eukaryotic cells. As it is difficult to visualize the operation of cells at the molecular level in real time, scientists typically rely on static electron micrographs to see the morphology of organelles. As a result, the dynamic operation of these organelles can sometimes be unclear.

Cisternal Maturation Hypothesis

In the cisternal maturation hypothesis, the cisternae of the Golgi apparatus evolve. Proteins leave the endoplasmic reticulum, and enter the cis-Golgi. The cisterna of the cis-Golgi then matures, with its enzymatic contents and internal environment changing as it becomes the medial-Golgi, and, eventually, the trans-Golgi.

In this model, the proteins never physically leave their membrane-bound cisternae during their transit across the Golgi. Instead, the entire unit of contents remains within the evolving cisternae.

Vesicular Transport Hypothesis

In contrast to the cisternal maturation hypothesis, the vesicular transport hypothesis posits that the cis-, medial-, and trans-Golgi cisternae are more static structures. Instead of evolving around their contents, the contents are physically shuttled via vesicular intermediates from each cisterna to the next.

In the case of vesicular transport, vesicles are shuttled along microtubules. Motor proteins facilitate this movement, with unique proteins being used for each direction of movement along a microtubule.

Microtubules are closely associated not only with vesicular transport, but also with cilia and flagella physiology. Which statement is true regarding cilia and flagella?

I. Most ciliated cells have many cilia structures

II. Most flagellated cells have only one flagellum

III. Only flagella have a 9+2 microtubule arrangement

IV. Both cilia and flagella move via the action of dynein

Possible Answers:

I and IV

III and IV

I, II, and IV

IV only

I, II, III, and IV

Correct answer:

I, II, and IV

Explanation:

Most ciliated cells have many cilia, as their role is important in such efforts as moving mucous in the respiratory tract. Flagellated cells, in contrast, typically have one flagellum that is used for locomotion. Dynein is important in both structures, as is the 9+2 microtubule structure.

Example Question #4 : Other Cell Structures

There are two models for the operation of the Golgi apparatus in eukaryotic cells. As it is difficult to visualize the operation of cells at the molecular level in real time, scientists typically rely on static electron micrographs to see the morphology of organelles. As a result, the dynamic operation of these organelles can sometimes be unclear.

Cisternal Maturation Hypothesis

In the cisternal maturation hypothesis, the cisternae of the Golgi apparatus evolve. Proteins leave the endoplasmic reticulum, and enter the cis-Golgi. The cisterna of the cis-Golgi then matures, with its enzymatic contents and internal environment changing as it becomes the medial-Golgi, and, eventually, the trans-Golgi.

In this model, the proteins never physically leave their membrane-bound cisternae during their transit across the Golgi. Instead, the entire unit of contents remains within the evolving cisternae.

Vesicular Transport Hypothesis

In contrast to the cisternal maturation hypothesis, the vesicular transport hypothesis posits that the cis-, medial-, and trans-Golgi cisternae are more static structures. Instead of evolving around their contents, the contents are physically shuttled via vesicular intermediates from each cisterna to the next.

In the case of vesicular transport, vesicles are shuttled along microtubules. Motor proteins facilitate this movement, with unique proteins being used for each direction of movement along a microtubule.

In the Golgi apparatus, mannose-6-phosphate is often added to proteins to mark them for delivery to lysosomes. Which membrane-bound structure fuses with lysosomes following uptake of large and small extra-cellular particles, respectively?

Possible Answers:

Endosome and vacuole

Pinosome and endosome

Pinosome and vacuole

Vacuole and endosome

Endosome and pinosome

Correct answer:

Endosome and pinosome

Explanation:

An endosome is a specific type of intracellular vesicle formed from the uptake of large particles from the extracellular environment. Similarly, a pinosome is an intracellular vesicle formed from the uptake of small particles or fluids. Endosomes result from endocytosis; pinosomes result from pinocytosis.

These vesicles are transported to the lysosomes in the cell, where they fuse and deposit their contents into the lysosome structure for degradation by hydrolytic enzymes.

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